(1) Field of the Invention
The present invention relates to an optical modulator, and more particularly, to an optical modulator which can transmit a sub-carrier multiplexing (SCM) signal with a large output using an optical fiber.
(2) Related Art Statement
A broadcasting wave or a CATV wave is a SCM signal obtained by intensity-modulating a plurality of radio frequency (RF) carriers having an interval of 6 MHz based on a multi-channel (CH) video or audio signal. In order to transmit the SCM signal using an optical fiber, in a hybrid-fiber coaxial (HFC) system or a fiber-to-the-premise (FTTP) system, a direct modulation (DM) method of intensity-modulating light of a laser diode (LD) by the SCM signal according to the length of the fiber, the distribution number or the like, or an external modulation (EXT-M) method of intensity-modulating continuous light of a laser diode (LD) by the SCM signal using a lithium niobate (LN) modulator is employed.
In the direct modulation method, frequency variation (chirp) associated with the intensity modulation is large and a transmission distance is restricted by a complex second-order distortion generated by chromatic dispersion of the fiber. Meanwhile, in the external modulation method, a LN modulator is generally employed, the chirp is small and the modulator itself does not cause the CSO distortion, thereby performing more long-distance transmission.
FIG. 1 illustrates a configuration of a CATV transmitter (TX) employing a LN modulator.
Reference numeral 1 denotes a light source, reference numeral 2 denotes a LN modulator, reference numeral 3 denotes a LN modulation element, reference numeral 4 denotes a phase modulation unit, reference numeral 5 denotes an intensity modulation unit, and reference numeral 6 denotes a DC phase adjustment unit for setting an operation point of the intensity modulation unit.
As the light source 1, a DFB laser having low phase noise (RIN noise) and a small line width is used. As a wavelength band thereof, a band of 1.3 μm has been much used, but a band of 1.5 μm has become mainstream along with the popularization of an EDFA (Erbium-Doped Fiber Amplifier).
Although the below description will be made based on the band of 1.5 μm, the band of 1.3 μm is similar except the EDFA. The output power of the DFB laser 1 becomes stable by an APC (Automatic Power Control) circuit 7. A micro wave (SCM-RF signal) related to the SCM signal is applied to the intensity modulation unit of the LN modulator 2 through a driver 9. At this time, an operation point which is a center of the modulation operation is controlled through an ABC circuit 10 and set to an intermediate point (Quad) of p-p intensity of a modulation output waveform by adjusting a DC voltage (bias) applied to the DC phase adjustment unit 6. In addition, the ABC circuit monitors light output from the LN modulator.
The SCM-RF wave from a SCM-RF signal source 12 is a RF signal obtained by modulating carriers (CH carriers) having an interval of 6 MHz using a VSB method in an analog video and using an OFDM and 64QAM method in a digital video over 70 MHz to 860 MHz.
By the intensity modulation, the output light spectrum becomes modulation spectrum (DSB modulation wave) of f0±(70 MHz to 860 MHz) at the both sides of a light source carrier having a frequency f0 and power P0 as shown in FIG. 2. Here, a sign (+) is referred to as an upper side band (USB) and a sign (−) is referred to as a lower side band (LSB). In addition, the intensity of the modulation spectrum depends on the intensity of the RF signal of each CH carrier, and the intensity of the spectrum is denoted by Pr.
An intensity ratio Pr/P0 is a modulation index per each CH carrier and is generally about 3% because of the linearity of the LN modulator. In this case, the spectral band width (FWHM) of the output light (an input light frequency component and a carrier component P0) is the same as the spectral width of the light source, and SBS (Simulated Brillouin scattering) is induced at about 9 dBm when the output light is input to a 1.3-μm SSMF (Standard Single Mode Fiber) which is a general transmission medium, and, although the output of the light input to the fiber more increases, an increment is not transmitted and returns to an input side.
A power threshold value for inducing the SBS varies depending on the fiber or the spectral width of the light source. A general DFB laser has a line width of about 3 MHz. In this case, the SBS threshold value is about 9 dBm (Pn). When an equivalent spectral width of the light source increases by any means, the SBS threshold value (Psbs) increases with respect to the increment ΔνD, as expressed by Equation 1.Psbs/Pn=10·log(1+ΔνD/ΔνB)  (1)
Where, ΔνD denotes a Brillouin gain width of the fiber and is about 65 MHz in the SSMF. For example, if ΔνD is 860 MHz, the SBS threshold value increases by 11.5 dB, that is, the SBS threshold value becomes about 20 dBm.
In order to increase ΔνD, conventionally, as shown in FIG. 1, a method of providing the phase modulation unit 4 in the LN modulation element 3, amplifying a signal from a single frequency signal source 11 having a frequency fm using an amplifier 8 to drive the phase modulation unit 4, strongly modulating a phase having a modulation index of at least two, spreading out the spectrum of the light source, and intensity-modulating the spread spectrum of the light source by the SCM-RF signal is employed.
The spectrum of the modulation wave in this case is shown in FIG. 3. Here, fm denotes a frequency which does not cause overlap between the adjacent USB and LSB. In general, fm may be 2 GHz. The light source carrier f0 is magnified to f0±fm, f0±2fm, . . . by the modulation, as shown in FIG. 3, and, as shown, each magnified light source carrier has modulation spectrums of the modulation waves USBn and LSBn by the SCM-RF signal, respectively (n is an integer).
The sizes of the respective light source carriers are defined as Jn2(m) and the distribution thereof varies depending on a phase modulation index m. For example, if a value m having the substantially same size until n=2 is employed, the light source spectral width is about 8 GHz and the SBS threshold value increases by about 100 times based on Equation 1.
According to the method shown in FIG. 1, SBS resistance increases and high-output light can be input to the transmission fiber, but a failure due to the magnification of the spectral width occurs. In other words, when a 1.3-μm single mode fiber (SMF) is used in the band of 1.5 μm, a transmission delay difference occurs between the modulation waves USBn and LSBn by the chromatic dispersion characteristics of the fiber and a detection RF wave is distorted in a square-law detection using a conventional photodiode (PD). The CSO distortion in this case is disclosed in Non-patent document 1 and is expressed by Equation 2.CSO (dB)=10·log {Ncso[⅜(λ2/2πC·DL)2·Σmpm2(fm)4 ·mi]2}  (2)
Where, Ncso denotes the number of evaluated CHs, mpm denotes a phase modulation index of the phase modulation unit, mi denotes an intensity modulation index, λ denotes an optical center wavelength, C denotes a vacuum light speed, D denotes the chromatic dispersion of the fiber and is about 17 ps/nmkm, L denotes the length of the fiber, and Σ denotes the sum of the light carriers.
[Non-patent document 1] M. R. Phillips et. al. “Chromatic dispersion effects in CATV analog light wave system using externally modulated transmitters” Optical Fiber Communication '96 Postdeadline papers 17-2
As expressed by Equation 2, the CSO distortion significantly increases by mpm and fm. In order to maintain a defined value (−65 dBc) (dBc is a value of the carrier), the transmission distance L must be reduced. In other words, when the modulation degree and the modulation frequency of the phase modulation unit increase in order to increase the SBS resistance, the transmission distance is restricted by the chromatic dispersion of the fiber.
Furthermore, mpm is about 2 and fm is about 2 GHz, but, in this case, a radio frequency amplifier having an output of a few W (Watts) is required and low power consumption and downsizing of the system cannot be realized.
Meanwhile, as described in Patent document 1, a method of adjusting an output of a light source carrier to improve a ratio between the outputs of the light source carrier and a sideband is disclosed. FIG. 4 illustrates a configuration of the method. Light from a light source 20 is branched into two light waves by a branch unit 21. One light wave is phase-modulated at the same frequency by a phase modulator 22 and an intensity modulator 23, and is DSB-intensity-modulated by adjusting a relationship between a modulation index and a phase. The other light wave is phase-shifted by an optical phase shifter (adjuster) 24. Then, the interferences of the both light waves are multiplexed by a multiplexer 25 and interference light is then output as modulation light.
[Patent document 1] Japanese Unexamined Patent Application Publication No. 2001-159750
The variation of the light spectrum generated by the above-mentioned configuration is shown in FIG. 5. The light spectrum from the light source 20 is a single mode light, as shown in FIG. 5A. When the light passes through the phase modulator 22, the single light source spectrum is divided into a plurality of light source carriers, as shown in FIG. 5B, and the respective light source carriers are intensity-modulated by the intensity modulator 23, thereby generating sideband components between which each light source carrier is sandwiched, as shown in FIG. 5C. Meanwhile, the other branched light wave branched by the branch unit 21 is shifted by the phase adjuster 24 in an opposite phase state, as shown in FIG. 5D. When the both light waves are synthesized by the multiplexer 25, the light output of the central light source spectrum is reduced, as shown in FIG. 5E.
When comparing the light wave emitted from an optical modulation system shown in FIG. 5E with the modulation light spectrum using the conventional phase modulator and intensity modulator shown in FIG. 5C, the intensities Pr of the USB and LSB are identical to each other, but a light source carrier component P0′ is greatly less than P0, and Pr/P0′, which is the modulation index m, more increases than Pr/P0. In addition, since a modulation curve of a LN intensity modulator has sine wave characteristics, when RF intensity modulation is performed focusing around a Quad point, an odd functional modulation is performed and a second-order distortion component does not appear.
When the size of the RF signal increases, the modulation curve is deviated from a straight line and thus a third-order distortion component and an odd-order harmonic component are output. In the LN modulator, the modulation index is about 0.15 and a CTB (Composite triple beat: third-order distortion) reaches the defined value (−65 dBc). In general, m per CH is about 0.03, the number of CHs is 10 CH, and a total modulation index mt in this case is about 0.3. In addition, since the CTB in this case is about −40 dBc, a method of providing a distortion correction circuit 13 between the signal source 12 and the driver 9 to previously distort the RF signal with an opposite polarity of the distortion of the intensity modulator is generally used, as shown in FIG. 1.
In the configuration shown in FIG. 4, as shown in FIG. 5E, it is possible to keep Pr in a linear intensity modulation range of the LN modulator and to reduce the light carrier to maintain the modulation index of about 0.3. In other words, it is possible to realize a system in which the distortion correction circuit 13 is unnecessary.
However, when adjusting P0′ which is the light source carrier component, a branch ratio of the branch unit 21 is very important, but, in Patent document 1, adjustment of the branch ratio is not described. In addition, as an effect, only distortion suppression is described and fiber dispersion affecting the transmission distance is not considered.
Furthermore, as mentioned above, the CSO distortion significantly increases by the modulation frequency fm in the phase modulation unit 22, but reduction of fm is not considered.
The present invention is directed to solve the above-mentioned problems. Accordingly, an object of the present invention is to provide an optical modulator which magnifies a modulation index and reduces affection of SBS by suppressing a light source carrier component in an equivalent light source spectral width and performs long-distance transmission by suppressing one side of sideband spectrum generated by a modulation signal to reduce affection of fiber dispersion.
In addition, another object of the present invention is to provide an optical modulator in which these functions are incorporated on one substrate and which can reduce the number of peripheral circuits and have excellent cost performance.